Fluid devices, such as fluid pumps, typically include a displacement assembly (e.g., a rotor assembly, cylinder barrel assembly, gerotor assembly, etc.) that displaces a certain volume of fluid as the displacement assembly rotates about a rotational axis. Of these fluid devices, many are of the types that include rotors with fluid pumping elements that reciprocate radially relative to a rotational axis (e.g., vane type, radial piston type, cam-lobe type, etc.). These fluid pumping elements act against a cam surface. As the rotor rotates about the rotational axis, the fluid pumping elements extend and retract in response to the rise and fall of the cam surface. This extension and retraction of the fluid pumping elements results in fluid being pumped through the fluid device.
These types of fluid devices can be fixed displacement devices or variable displacement devices. In the variable displacement devices, the displacement is typically varied by offsetting the rotor relative to the cam surface. Such an offset can increase or decrease the distance traveled by the fluid pumping elements thereby increasing or decreasing the volume of fluid displaced through the fluid device.
While these types of fluid devices work effectively in many different applications, some applications require variable fluid devices having higher efficiency ratings. One type of fluid device that is credited with higher efficiency ratings uses a flexible band that surrounds the rotor and the pumping elements. The pumping elements act against the flexible band to pump fluid. In order to change the displacement of the fluid device, the shape of the flexible band is changed, displaced or deformed. However, as a result of the deformed shape of the flexible band and as a result of the pumping elements acting against the flexible band, stresses develop within the flexible band. Often these stresses can decrease the life of the flexible band.